As a technology for realizing communications of large-volume data such as images, MIMO (Multi-Input Multi-Output) communications are being studied actively in recent years.
Among them, BLAST (Bell Laboratories Layered Space-Time) in particular is a focus of attention as an application capable of realizing high-speed transmission using an MIMO channel. This is a technique for transmitting mutually independent (or coded) streams from a plurality of transmission antennas and detecting the respective streams while repeating spatial filtering and removal of replicas on the receiving side.
Furthermore, when the MIMO channel information is known to the transmitting side, it is known that a greater channel capacity can be obtained. More specifically, this is realized by carrying out directivity control using an eigen vector obtained through singular value decomposition of a matrix which consists of the respective channel responses of the MIMO channels as elements and forming a spatially orthogonal channel (eigen channel). That is, when the MIMO channel information is known to the transmitting side, it is possible to form an orthogonal channel through multi-beam formation using the eigen vector, performing transmit power control through an irrigation theorem and thereby maximize the channel capacity (e.g., TECHNICAL REPORT OF IEICE RCS2002-53 (2002-05), Institute of Electronics, Information and Communication Engineers).
When the above described technology is applied to an actual apparatus, radio transmission is carried out after preparing a plurality of transmission systems capable of carrying out transmission processing on a plurality of transmission data streams and assigning weights by multiplying the transmission signals by their respective complex weights (hereinafter simply referred to as “weights”).
When a bit error rate on the receiving side does not satisfy a predetermined value, an automatic retransmission request (ARQ; Automatic Repeat reQuest) is also generally practiced whereby the receiving side sends a retransmission request signal to the transmitting side and the transmitting side retransmits the same transmission data in response to this request.
Especially packet transmission which transmits data traffic is required to guarantee error-free data transmission, and therefore error control through ARQ is indispensable. In addition, when an adaptive modulation and error correction intended to improve the throughput by selecting an optimal modulation system or coding system according to the condition of a propagation path (path) are applied to packet transmission, it is not possible to avoid measuring errors or packet errors caused by a control delay, etc., and therefore the 3GPP (3rd Generation Partnership Project) also standardizes the use of a hybrid ARQ (hereinafter referred to as “HARQ”) which incorporates an FEC (Forward Error Correction) function.
Therefore, by carrying out an MIMO communication using a plurality of antennas during data transmission to realize a large-volume data communication and retransmitting data when received data contains errors on the receiving side and by combining received data at the time of initial transmission with the received data at the time of retransmission using HARQ on the receiving side, a considerable improvement of throughput can be expected for this radio communication system.
However, even when received data contains errors and data is retransmitted, if a time variation of an environment of a propagation path which a transmission signal follows is slow (see FIG. 1), for example, when the communication apparatus is at rest or moving at a low speed, the diversity gain obtained with reception power on the receiving side is small, and therefore there is a problem that the throughput of the radio communication system hardly improves even if data is retransmitted.
This is because when the time variation of the propagation path environment is slow, a signal whose reception level is low at the time of initial transmission also has a low transmission level at the time of data retransmission, and therefore data cannot be demodulated correctly even if the data at the time of initial transmission and data at the time of retransmission are combined. Furthermore, when a multi-antenna technology such as MIMO or STC (Space-Time Coding) is used, if the time variation of the propagation path environment is slow, there is little variation in the fading condition between initial transmission and data retransmission, and the combined data cannot be demodulated correctly.